The present invention relates to a filling composition for optical fiber cable and articles comprising the composition, particularly telecommunication cables, especially optical fiber cables.
Materials which are placed around optical fibers in optical fiber cable to provide protection against water ingress are commonly referred to as filling compounds. These compositions must possess a number of properties in order to provide protection against water without adversely influencing fiber transmission properties, often referred to as attenuation. The permissible range of these properties will also be affected by the cable design and the coatings used on the optical fiber. However, there are general principles which apply independent of cable design.
First, as discussed in U.S. Pat. No. 4,701,016, these compositions must have low critical yield stress (be very soft) in order to prevent attenuation of the fiber by a mechanism known as "microbending". Because cables are often exposed to low temperature, typical cable designs require satisfactory performance down to -40.degree. C. For instance, with respect to low temperature stiffness, it has been shown for a slotted-core cable design that penetration as measured by ASTM D217 at -40.degree. C. must be greater than about 180 dmm to give satisfactory fiber attenuation (T. Hattori et al., Proceedings of the International Wire and Cable Symposium, 12-15 (1988)). The critical yield stress of filling compositions for optical fiber telecommunications cable is generally measured at room temperature.
A second requirement with respect to attenuation is related to the swelling of the coating materials on the fiber by oils in the filling composition. This problem is generally associated with the primary (inner) coating on a dual coated optical fiber. Such swelling must be limited or the result may again be an increase in attenuation due to microbending.
Another requirement for filling compositions is that they remain in the cable (not drip out of the cable) at temperatures up to 80.degree. C. Two of the most widely used tests in the industry to determine cable compound flow or drip are the Bell Communications Research (Bellcore) test (Bell Communications Research, Inc., Technical Reference TR-TSY-000020, Issue 4 (March 1989) and the Rural Electrification Authority (REA) test (Rural Electrification Administration, "Specification for Totally Filled Fiber Optic Cable--PE-90" (May 28, 1986). Further, the tendency for oil to separate from the filling compound or gel has been found to correlate to drip of the composition from a cable. Two oil separation tests have been reported by the industry: one is a centrifuge test at ambient temperature (U.S. Pat. No. 4,701,016) and the other is a high temperature oven test reported in M. C. Light, Jr., International Wire & Cable Symposium Proceedings, 459-464 (1988). Because these two tests were developed by manufacturers of different cable designs, there may be different passing criteria for different cable designs. For a design referred to in the trade as a loose tube or buffer tube, passage requires less than about 15% oil separation at 80.degree. C.; this corresponds to no drip from the cable at 80.degree. C. No industry standard has been published for the centrifuge test.
To provide the necessary yield stress properties, filling compositions are generally thixotropic greases such as those formed by blending an oil with a fumed silica and, optionally, a thermal oxidative stabilizer. The use of a hydrophobic fumed silica to gel a mineral oil is disclosed in European Patent Application Publication No. 0067009; the use of a hydrophobic fumed silica to gel polybutene having an average molecular weight of about 250 to 500 is disclosed in U.S. Pat. No. 5,050,959.
To be acceptable with regard to high temperature drip, the oils must not separate from the fumed silica/oil/stabilizer blend. To accomplish this end, high molecular weight polymers or 90.degree. C. softening point waxes, such as those disclosed in U.S. Pat. Nos. 4,701,016 and 4,370,023, have been added to the grease to act as thickeners. Unfortunately, addition of the thickeners of these types also increases the stiffness, which may introduce microbending. In addition, dissolution of these polymers in the oil often requires a high temperature blending operation, which can add significantly to the cost of the filling composition.
In order to obtain compatibility with the fiber coatings, synthetic oils, such as poly .alpha.-olefins, have been used. An acceptable molecular weight range is disclosed in T. Hattori et al., Proceedings of the International Wire and Cable Symposium, 12-15 (1988). These oils, such as polyalkenes, also have inherently low pour points and, therefore, also produce materials which are capable of remaining soft at low temperature.
By far the most difficult set of properties to obtain are low critical yield stress and acceptable low temperature stiffness while retaining low oil separation necessary to prevent drip of the filling composition from the cable at 80.degree. C. Prior art formulations have not been able to obtain, at the same time, both satisfactory low temperature performance with respect to attenuation and high temperature performance with respect to the composition not dripping out of the cable. Thus, there exists a need for an improved filling composition that is capable of remaining in a cable at high temperatures yet able to prevent unnecessary "micro-bending", particularly at low temperatures.
Throughout this application, various publications are referenced. The disclosures of these publications in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of the art to which this invention pertains.